1. Technical Field
The present invention relates to an optical coupler and, more particularly, to an optical coupler which is insensitive to the polarization state of a message signal passing therethrough.
2. Description of the Prior Art
There exist many applications where it is desirous to multiplex optical signals at different wavelengths onto the same optical signal path. One application of current interest is in fiber-based optical amplification, where it is required to multiplex an optical message signal at a first wavelength .lambda..sub.S with a "pump" signal at a different wavelength .lambda..sub.P within an optical gain medium (such as an erbium-doped optical fiber) to provide amplification of the message signal. For the erbium-based arrangement, which appears to be the most commercially viable at this point in time, the pump source must operate at about 0.98 .mu.m or 1.48 .mu.m to provide amplification. Laser sources available for emitting radiation at either of these wavelengths are known to be relatively low in power, thus limiting the amount of amplification which can be obtained from a single pump source. A solution to this problem is to use multiple pumps, which necessarily increases the size of the resultant amplifier. Further, each pump laser often requires its own isolator, coupler, etc., which may introduce intolerable losses into the system.
Another application of multiple pump sources is the resultant increase in reliability of the amplifier. That is, for systems using only a single pump laser, failure of the pump results in complete loss of amplification. Therefore, most applications recommend the use of (or availability of, at least) additional pump sources.
There exists in the prior art an optical coupling arrangement for providing a plurality of pump sources with a message signal to increase the reliability of an associated optical coupler. U.S. Pat. No. 4,805,977, issued to Y. Tamura et al. on Feb. 21, 1989 discloses an optical coupler comprising a polarization beam combiner (i.e., polarization multiplexer) responsive to a pair of orthogonal (in polarization state) pump signals. The output from the polarization multiplexer provides a pump signal of increased power (with respect to that associated with a single pump) which is subsequently applied as an input to a wavelength division multiplexer for combining with an applied message signal. Although the Tamura et al. arrangement does provide increased pump power, the system remains sensitive to the polarization state of the received message signal. In most applications, the polarization state of the message signal is not known and may even vary as a function of time.
Therefore, a need remains in the prior art to provide a multiplexer, suitable for fiber amplifier applications, which is insensitive to the polarization state of the message signal and compatible for integration with an optical isolator.